This invention relates to multizone electrical furnace systems, and more particularly to methods and apparatus for controlling such systems.
Many electrical furnaces have a plurality of thermal zones which are separately controlled, for example, to provide uniform temperatures throughout the furnace under various operating conditions. One example of furnaces of this type are high pressure, high temperature furnaces used as hot isostatic presses. Because of the large power requirements of such furnaces, polyphase (usually three phase) power is typically supplied to the furnace. The heating element or elements for each thermal zone are connected in circuit relation with a respective one of the phases of the power supply, i.e., the heating elements for each thermal zone are connected either between a pair of power supply lines (so-called phase to phase or delta connection) or between a power supply line and ground (so-called phase to ground or Y connection). If there are more thermal zones than power supply phases, the heating elements for two or more thermal zones are connected in circuit relation with each phase of the power supply. The amount of power supplied to the heating elements for each thermal zone is controlled by a thyristor device (e.g., a semi-conductor controlled rectifier or Triac) connected in series with the heating elements for that thermal zone. The thyristor devices control the amount of power supplied to the associated heating elements by clipping or suppressing a portion (typically the initial portion) of each oscillation of the power supply signal. The effect of such clipping is to shift the phase of the power applied to the heating elements relative to the phase of the power supply signal.
It is frequently necessary to supply substantially different amounts of power to the several thermal zones of the furnace. In a vertically disposed hot isostatic press having vertically adjacent thermal zones, for example, temperature uniformity is achieved at low pressures by supplying approximately the same amount of power to each thermal zone. At higher pressures, however, convection becomes an increasingly important heat transfer mechanism, and substantially more power must be supplied to the lower thermal zones than to the upper thermal zones to maintain temperature uniformity. When substantially different amounts of power are supplied to the various thermal zones, substantially different phase shifts are produced in the power supplied from the various power supply phases by the action of the thyristor devices. These different phase shifts can cause excessive currents in the power supply lines. These currents can be significantly greater than would be expected without considering the phase shift effect of the thyristor devices. The result of these excessive line currents can be overheating of and damage to the external circuits (e.g., the power supply lines).
Another problem encountered in the operation of electrical furnaces is that the heating elements in the furnace dissipate much more power at low furnace temperatures than they do at high furnace temperatures. Particularly when the power supply is designed to provide rapid warm-up of the furnace, the heating elements may be substantially over-powered when the furnace is at higher temperatures. This problem can be especially serious in furnaces with molybdenum heating elements which are capable of absorbing more power at higher temperatures because of the substantially increased resistance of molybdenum at those temperatures. If the heating elements are substantially over-powered (i.e., the heating elements are absorbing more power than they can dissipate), the heating elements may overheat and burn out.
In view of the foregoing, it is an object of this invention to provide improved electrical furnace systems.
It is a more particular object of this invention to provide electrical furnace systems in which the effects of the phase shifts produced by the thyristor control devices are substantially reduced.
It is another more particular object of this invention to provide electrical furnace systems in which the match between the power supplied to the furnace heating elements and the power dissipating properties of those elements at various temperatures is improved and over-powering of the heating elements at higher temperatures is substantially prevented.